Although the initial publication of the dielectric resonator dates back to 1939, its development and application have been held back due to temperature sensitivity. Martin Stiglitz, in his article entitled "Dielectric Resonators: Past, Present, and Future", Microwave Journal, vol. 24, pp 19-36, July 1981; observes a number of breakthroughs in dielectric resonator materials that have served to reduce the effects of temperature.
With the development of high dielectric constant, high Q, low loss, temperature compensated ceramic materials, the dielectric resonator stabilized oscillator (DRO) has become a practical, useful device. However, dielectric resonator oscillators still experience the problem of frequency drift with changes in temperature.
The task of compensating for the temperature sensitivity of dielectric resonator oscillators has been reduced, to some extent by the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 4,489,289 issued to Slobodnik et al; PA0 U.S. Pat. No. 4,380,745 issued to Barlow et al; PA0 U.S. Pat. No. 4,386,326 issued to Yoshimura; PA0 U.S. Pat. No. 4,415,870 issued to Zumsteg; PA0 U.S. Pat. No. 4,427,952 issued to Zumsteg; PA0 U.S. Pat. No. 4,445,097 issued to Godart et al; and PA0 U.S. Pat. No. 4,454,483 issued to Baylor.
Slobodnik et al disclose a method of digital temperature compensation of surface acoustic wave (SAW) oscillators. This involves the use of two oscillator paths on the same SAW substrate, one of which serves as a temperature sensor.
Godart et al disclose a microstrip transistor oscillator with dielectric resonator stabilization. The patented resonator has a very low temperature coefficient and is used at a frequency of 3 to 10 GHz. Yoshimura discusses an 11-12 GHz oscillator with a dielectric resonator of a large dielectric constant and a high Q factor. Barlow et al show a digitally controlled temperature compensation circuit for a crystal oscillator. The correction signal is generated by a digital frequency synthesizer controlled by a programmable read-only memory. A program in the memory generates the required correction frequency for each temperature code over the operating temperature range. The temperature code is generated by gating a digital counter with the output of a monostable multivibrator which utilizes a thermistor to make its gate interval proportional to temperature. In Baylor temperature compensation of an oscillator is by a fractional cycle synthesis. A digital signal representing temperature is generated by an analog-to-digital converter from a temperature sensor. A signal representing the relationship between crystal frequency and temperature is stored in a memory and after appropriate treatment is injected into a phase locked loop containing a voltage-controlled oscillator. The two Zumsteg patents are both directed to oscillator circuits with digital temperature compensation in which the correction factor is stored in a PROM.
The following references are pertinent to the present invention:
Frerking, M. E. Crystal Oscillator Design and Temperature Compensation, Van Nostrand, New York 1978;
Lee, J., J. E. Andrews, K. W. Lee, and W. R. Day, "Digital and Analog Frequency - Temperature Compensation of Dielectric Resonator Oscillators," IEEEMTT-S Digest, pp 277-279, 1984.
The Frerking reference contains a general discussion of temperature compensation in the context of bulk mode devices and generally with temperature sensing external to the crystal.
The publication of Lee et al contains a description of a system which provides digital temperature compensation to dielectric resonator oscillators.
Also, Varian has advertised (Microwave Journal, May 1984, page 21) a commercial temperature compensation system having an average temperature stability of 0.36 ppm/.degree. C.
Our solution to the problem of temperature correction is an approach which allows temperature compensation to be performed over a tunable frequency range by providing temperature sensitive phase shift feedback to dielectric resonator oscillator loops. The present invention is intended to provide this solution in the form of digital temperature correction to a phase shift circuit in a dielectric resonator oscillator loop.